The invention concerns a method and a device for determining the surface form or shape of biological tissue.
The exact knowledge of the topology of biological tissues is in many instances indispensable, e.g., for carrying out operations of the tissue surface. The corneal surface of the human eye is cited as an example. Since the cornea has a power of refraction of above 40 diopters, it is decisive in the refraction of the light falling into the eye and thus participates in the seeing process. The power of refraction thereby is primarily a function of the form of the corneal surface and in particular of its curvature. Modern methods of correcting ametropias therefore aim to alter the corneal form by the removal of corneal tissue with the aid of a laser. Therefore, the prerequisite for a purposeful working of the cornea is the exact knowledge of the form of its outer surface. This is currently determined before and several days after the correction of the ametropia with the aid of optical methods in which the measured values are not appreciably influenced by the statistical and involuntary movements of the eye on account of the rapidity of these methods.
A known method of measuring the corneal form, which is used before or after an ametropic operation or also in order to adapt contact lenses, is based on the use of so-called keratometers, in which the reflection of concentric rings (so-called placido rings) on the tear film that moistens the cornea is recorded with a camera and evaluated. An illuminating device is placed in front of the eye and a disk with circular slits concentric to each other is arranged in front of the device and in whose center a camera is set up. The light reflected from the tear film and recorded by the camera in the form of a ring pattern distorted by the curvature of the cornea is compared in order to determine the specific characteristics of the corneal form to be measured with a given corneal form of a standard eye with a corneal radius of 7.8 mm. In order to reconstruct the surface form of the particular cornea, the user first manually determines the center of the rings, usually approximately 20, with the aid of cross hairs. 180 meridians are then placed through the center over the cornea with an interval of 1xc2x0 each. The interval of the intersections of the meridians with the rings increases with the growing radius of the rings up to values of approximately 300 xcexcm. Altogether, 180 (meridians)xc3x9720 (rings)xc3x972 (intersections)=7200 data points result from which the curvature of the cornea can be calculated. This known method and this known device have the disadvantage that due to the concentric arrangement of the illuminating device and of the camera, no data can be recorded in a surface of the center with a diameter of at least 1.5 mm. However, measurements are especially important particularly in this area. Furthermore, erroneous measurements of a corneal form cannot be avoided which form deviates greater than is customary from the form of a standard eye, such as, e.g., in the case of a central flattening. In addition, the number of 7200 data points is insufficient in some instances for the interpolation necessary to determine the corneal topology. This number of data points effectively represents an upper limit since the meridians cannot be divided at an angular interval of less than 1xc2x0 on account of their finite width of line.
Since the previously described method and the corresponding device do not permit any monitoring during the removal process, erroneous corrections are recorded relatively frequently, especially in the case of high ametropias above xe2x88x926 diopters. These erroneous corrections can be evaluated by the user or the operator statistically to prepare so-called xe2x80x9cnomogramsxe2x80x9d that aid in preventing the erroneous corrections in the means in subsequent operative incisions; however, this solution is not adequate.
Moreover, the industrially established so-called strip-projection method for the optical measuring of surfaces of very different types of lifeless materials is known that permits a reliable, contactless and rapid detection of measured values. The basic idea of the strip-projection technique resides in the uniting of measuring-technology possibilities of geometrical-optical triangulation with those of classic interferometry. The mathematical connections are presented in detail in the annex. This method and the corresponding device are particularly suited for detecting rapid events since only a single photograph is necessary. In this method a suitable strip pattern is first projected onto the surface to be measured. The strips are generated by interferometry or by the representation of a suitable structure (grid, etched structure in glass, LCD matrix, micromirror). The light diffusely scattered from the surface in the form of a strip pattern distorted by the surface form of the cornea is detected at an angle a to the direction of projection or irradiation and evaluated by suitable algorithms. The required Fourier transformations that were previously time-intensive no longer constitute an appreciable delay on account of new computer possibilities.
However, the evaluation of the strip patterns becomes problematic given a relatively weak contrast of the detected strip pattern. Phase-measuring errors occasionally occur that make themselves noticeable in jumps in the surface. As is known, contrast-elevating measures consist in vapor-depositing strongly scattering layers on the object or in the addition of fluorescent dyes. The latter has been suggested in particular in ophthalmology, e.g., by Windecker at al. (Applied Optics 43, 3644 ff., 1995) who suggested enriching the tear film in front of the cornea with fluorescein in order to determine the form of the corneal surface in a strip-projection method. In this method, blue light filtered with a filter out of white light is guided onto the cornea, whereupon the tear film located in front and enriched with fluorescein emits green light as a consequence of the excitation. U.S. Pat. No. 5,406,342 teaches a similar method (and a corresponding device) in which the superpositioning of two partial patterns projected from two directions onto the cornea provided with fluorescent liquid results in the production of a moire pattern that can be evaluated. The fluorescent radiation emitted by the liquid film is combined after having passed through an optical filter by the successive recording of two half images with a video camera and evaluated by specially developed algorithms. The projection of the radiation from two directions helps, in addition to the filter, to avoid the detection of the direct reflex that is produced at the location on the cornea whose surface normal divides the angle between the direction of radiation and the direction of observation into two equally large angles and always appears when the detection unit is sensitive to the wavelength projecting the pattern.
Other methods and devices for determining the corneal topography in which a fluorescent agent is applied onto the eye are known from U.S. Pat. No. 4,995,716; U.S. Pat. No. 4,761,071 and U.S. Pat. No. 5,159,361.
These known methods and devices have the disadvantage that the tear film always exhibits locally and individually different thicknesses so that conclusions about the surface of the cornea can not be reliably drawn from measuring it. Since the fluorescent agent continues to be distributed in the tear film and thus supplies scattered light from the entire thickness of the tear film, the measuring accuracy can not be greater than the film thickness, that amounts up to 200 xcexcm. Furthermore, the liquid would penetrate into the corneal tissue if the epithelial layer on the cornea were not present or folded back out of the radiation path, which would result in a widening of the depth resolution. Moreover, in such an instance the surface form of the cornea would change since it swells up. Thus, an intact epithelial layer is necessary for the use of this known method or this known device; however, it is precisely this layer that must be removed before an operation, so that measurements during an operation, for example, are not possible.
Moreover, GB-A-2,203,831 teaches a device for investigating tumors with fluorescent radiation. To this end the tissue (tumor) to be investigated is irradiated by a light source that emits light in the UV range. The tissue is excited as a consequence thereof to emit fluorescent radiation that is detected by a detection system and subsequently measured. However, the known device is only suited for investigating the tissue qualities of biological tissue such as tumors but not for determining the surface form of biological tissue.
Objects and advantages of the present invention will be set forth in the following description, or may be obvious from the description, or may be learned through practice of the invention.
The present invention addresses the problem of further developing a device of the initially cited type in such a manner that the topology of a biological tissue can be determined in a simple manner and in the absence of any liquids, in particular fluorescent liquids, and that the results can possibly be used for operative treatment.
This problem is solved in the method according to this invention of the initially cited type in that the tissue is directly irradiated with an irradiation pattern produced with the aid of an excitation radiation so that the irradiated tissue areas are excited to emit a fluorescent pattern consisting of fluorescent radiation, which pattern is detected and evaluated in order to calculate the surface form of the tissue.
Furthermore, the problem is solved in a device according to this invention wherein the radiation source generates an excitation radiation with a wavelength located substantially in the ultraviolet (UV) wavelength range.
The advantages of the invention can be seen in particular in the fact that no film located in front of the biological tissue and enriched, if necessary, with a suitable substance is excited to the omission of fluorescent radiation but rather the biological tissue itself is. To this end, the intensity, and in particular the wavelength of the excitation radiation, is selected in such a manner that its penetration depth into the tissue is low and actually only the outermost tissue areas (e.g., 2-3 xcexcm) are excited to fluorescence. The fluorescent pattern to be detected corresponds thereby substantially to the radiation pattern projected previously onto the tissue, distorted by the possibly curved surface of the tissue to be measured as well as by the angle between the detection of observation and the direction of radiation. The non-irradiated tissue areas are not excited thereby to the emission of fluorescent radiation. Since an undesired spatial distribution of fluorescent material, such as, e.g., in the case of a fluorescent film of liquid on the tissue, does not occur, no measuring inaccuracies occur as a result. Likewise, a swelling up of the tissue due to such a film of liquid located in front is avoided.
Especially in the instance of the cornea, fluorescent liquids or liquids marking in another manner to be applied onto the eye can be eliminated in the case of the method and device in accordance with the invention. It is thus possible in a surprisingly simple manner to directly represent the corneal surface by fluorescent radiation without having to take the imprecise detour via a fluorescent film located in front.
According to the invention, the radiation source generates an excitation radiation with a wavelength located substantially in the ultraviolet wavelength range. In this instance the UV radiation penetrates only a few micrometers into the cornea (the cornea is transparent above the UV wavelength range up to the near infrared (IR)). Consequently, the fluorescent radiation emitted from the cornea stems substantially from the outermost tissue layer and thus represents its topology in a sufficiently exact manner. Furthermore, the measuring can take place in a sufficiently short time to avoid erroneous measurements due to eye movements. By means of the method and the device according to the invention it is also possible to measure, e.g., deformations of extremities or changes of the skin surface and other structural features of the skin (e.g., fingerprints). Beforehand, it may be necessary to remove disturbing objects in the optical path from the tissue surface, e.g., hairs. The form of the surface of the tissue to be measured can basically be shaped in any way; however, no coarse graduations should be present.
The evaluating unit for evaluating the fluorescent pattern of the fluorescent radiation preferably comprises a computer with a suitable analyzing software that uses, e.g., known mathematical methods. Such a known mathematical method for evaluating the fluorescent radiation is given in the annex.
Since the method and the device of the invention are based on the fluorescent qualities of the tissue itself, the method is also particularly suitable for detecting the topology during the refractive operation on the cornea during which no tear film and no epithelial layer, at least in the radiation path of the excitation radiation, is present. For example, the instantaneous tissue topology is determined sufficiently often before and during operation so that the next operation step can be coordinated with the actual result. This thus permits the controlling/regulating of the removal process during the operation with a laser, for example, in that one can switch between operating mode and the mode for determining the surface form of the cornea, so that a more exact correction is possible than previously as a result of the constant monitoring and the corresponding reacting. As a result thereof, even nomograms individually determined by the user from statistical investigations become superfluous, which were still necessary up to the present, especially in the case of large corrections above xe2x88x926 diopters.
It is especially preferable if the radiation source for determining the surface form of the biological tissue and that for the operative treatment of the tissue are identical. In this manner, a compact and relatively inexpensive device for determining the tissue topology and the operation of the tissue can be realized. In this manner, very precise tissue corrections can be carried out rapidly and simply with the aid of the method and device of the invention. The problem is therefore also solved in a method for supporting an operative intervention on a biological tissue in that the result of the evaluation is included in a regulating and/or controlling manner in the actual operative treatment of the biological tissue.
Furthermore, it is advantageous if the detection of the fluorescent radiation can take place with a device whose sensitivity for the excitation wavelength is a priori very low on account of the wavelength difference between the excitation radiation and the fluorescent radiation. A CCD camera that permits a locally resolved detection can be used to this end. If necessary, a camera sensitive in the ultraviolet range can also be used. Additionally or alternatively, the sensitivity of the particular detection device for the excitation wavelength can be reduced by a filter (color filter or polarization filter). In this manner, the direct reflex does not appear in a disturbing manner during the detection. It can not be avoided, just as in the known methods and devices. However, it is not detected on account of the sensitivity of the detection device located in another wavelength range and can therefore not cover over the desired signal. Thus, only a single exposure is required for the complete reconstruction of the surface of the biological tissue.
The fluorescent radiation emitted by the biological tissue is preferably detected at an angle different from the direction of the radiation which angle is, e.g., 45xc2x0. In this manner the fluorescent pattern is observed in a perspectively distorted manner so that, in particular, a curvature of the tissue surface can be measured in a more precise fashion. For example, the adjacent strips of a fluorescent-strip pattern appear more curved in such an observation on account of the perspective than in a frontal observation, for which reason more precise information about the course of curvature can be obtained.
If the direct fluorescent radiation is caught solely with a single detection device and the direction of irradiation and the direction of observation do not coincide, a perspectively distorted image of the fluorescent pattern is obtained, as discussed. Therefore, in the case of a very fine irradiation pattern and therewith fluorescent pattern that makes possible a very fine resolution of the tissue topology, lines located in the areas facing away from the direction of detection coalesce in an undesired manner and can then no longer be precisely resolved. Therefore, at least one further detection device can advantageously be used which is opposite, relative to the direction of irradiation, the first detection device and serves to detect the fluorescent radiation from a range of the biological tissue that cannot be precisely detected by the first detection device. Alternatively, a mirror suitably positioned in front of the biological tissue can also be used that reflects the fluorescent radiation from the side of the biological tissue facing away from the (single) detection device to this latter detection device. In this manner, e.g., two spatial half images can be recorded simultaneously (with two detection devices) or successively (with one detection device and one mirror) and appropriately combined for evaluation.
Additionally or alternatively, the biological tissue is irradiated from at least two directions in order to sufficiently detect tissue areas that are otherwise difficult to illuminate on account of the perspective distortion. It is advantageous to provide a symmetrical design of the device of the invention for this when measuring a cornea, in which instance, e.g., the two directions, one of projection and one of irradiation, enclose the same angle with a direction of observation running between them. Also, in addition to using several radiation sources, the use of several detection devices can be provided. Likewise, mirrors or other light deflection devices are available for illuminating and/or irradiating the biological object from several sides.
The biological tissue, and in particular the cornea, are preferably excited to fluorescence with wavelengths between 150 nm and 370 nm. Wavelengths shorter than approximately 150 nm can currently be generated with sufficient energy only with a high technical expense. Moreover, they generate a fluorescent radiation that would be difficult to detect with conventional technology on account of their wavelength, which is likewise only slightly longer. Wavelengths longer than approximately 370 nm, on the other hand, exhibit, at least in the case of the cornea for visible light, too great a penetration depth to limit the emission of fluorescent radiation to the outermost cellular layers and therewith assure the required precision of measurement. In addition, damage to the eye could occur in this special case.
Since the eye makes involuntary movements, so-called saccades, during the measuring of the cornea, it is advantageous to appropriately limit the irradiation time, preferably below 20 ms, since the detected fluorescent pattern could otherwise be distorted. New laser devices are capable of emitting pulses on the magnitude of femtoseconds, that can also be used if necessary.
The cited problem of the obligatory limitation of the irradiation time due to the eye movements can be circumvented in the method and with the device in accordance with the invention by the use of at least one eye tracker, with which longer irradiation times can be realized. The eye tracker records the courses of the movements of the eye, preferably during the irradiation, as a function of the time, that are included during the evaluation of the fluorescence pattern. The excitation radiation is readjusted with respect to the eye by means of the information obtained in this manner in order to realize longer irradiation times.
Furthermore, a determination of the position of the eye with an eye tracker that can absolutely correspond to the eye tracker indicated above is preferably provided before each irradiation with the irradiation pattern and after each detection of the fluorescent pattern. It is additionally or alternatively provided to the above that the position of the eye is determined during the irradiation with the irradiation pattern and during the detection of the fluorescent pattern with an eye tracker. The irradiation or detection is halted upon a change of the position of the eye during the irradiation with the irradiation pattern or during the detection of the fluorescent pattern and a new irradiation of the cornea of the eye with subsequent detection of the fluorescent pattern is carried out.
It turned out to be advantageous if the irradiation pattern is cast in short time intervals onto the tissue. A sufficient fluorescent intensity with simultaneous protection of the tissue from light-induced removal is realized with such an irradiation pulse series. In the case of rapidly switching optical elements, several hundred irradiation pulses can be applied within the given flash time. The repetition rate for each measurement, that is composed of an irradiation with subsequent detection of the fluorescent pattern, is preferably between 1 Hz and 1 MHZ.
In order not to permanently damage corneal tissue or other biological tissue by the excitation radiation, the projection of the geometric irradiation pattern is carried out with a sufficiently low fluence (energy/surface). This fluence should be less than 10 mJ for a circular surface area with a diameter of 10 mm. Likewise, phototoxic effects are avoided therewith. The energy of the excitation radiation is preferably between 1 xcexcJ and 1 J.
The radiation exciting the fluorescence is emitted, e.g., by an excimer laser, that is also used for the operative work on the cornea. For example, an ArF laser with a wavelength of 193 nm is used. Alternative laser devices that permit the emission of radiation pulses are, in addition to excimer lasers (ArF with a wavelength of xcex=193 nm, KrF with xcex=248 nm, XeCl with xcex=308 nm, XeF with xcex=351 nm) and nitrogen lasers (xcex=337 nm), also frequency-multiplied solid lasers (Nd YAG 5-fold with xcex=213 nm and 4-fold with xcex=266 nm and 3-fold with xcex=355 or alexandrite) or dye lasers pumped by such solid lasers. It is advantageous to select a wavelength at which the intensity of the fluorescent radiation is as high as possible since the demands on the detection device drop as a result thereof.
An alternative to a laser is the use of more economical flash lamps filled with gaseous mixtures containing xenon or deuterium. These lamps are preferably limited by suitable filters to the emission of UV radiation. Since the output performance of these flash lamps in the UV range is usually lower than those of lasers, higher requirements must be placed on the detection device if necessary.
The geometric irradiation pattern projected onto the biological tissue such as, e.g., the cornea, preferably consists of parallel strips with a sinusoidal, coss2, or square intensity course. The surface can be measured with a resolution of a few micrometers therewith, e.g., at a strip width and a strip interval of 100 xcexcm with suitable algorithms. Alternatively, a grid whose intersections are used for the evaluation, a perforated pattern, a pattern consisting of several concentric circular lines with lines emanating radially from the center and with lines arranged with the same angular interval, a moire pattern consisting of two line patterns or some other geometric pattern can be selected.
The means for producing the geometric irradiation pattern preferably comprise a mask with, e.g., parallel slits or regularly arranged perforations that are reproduced by irradiation on the tissue. The intensity losses are relatively slight in these patterns, which is particularly advantageous in irradiation systems with relatively low output.
As an alternative, substrates (e.g., glass) altered structurally areawise by suitable preparation can be used as means for producing the irradiation pattern. Such substrates, for example, may have areas of strong scatter or absorption alternating with unprepared areas of high transmission. Such substrates permit the generation of a sinusoidal intensity course.
The irradiation pattern can also be generated in an advantageous manner by known diffractive, optical elements such as, e.g., microlenses. The microlenses, whose diameter is, e.g., 100 xcexcm, are applied, e.g., in a close, regular arrangement on a transparent glass substrate that is placed in the radiation path of the excitation radiation exhibiting, e.g., a ray diameter of 8 mm. If the individual microlenses are designed as cylindrical lenses, strip patterns can be generated in this manner. Even other lens forms, e.g., semicircular, are possible and define a different irradiation pattern and therewith different fluorescent pattern. The microlenses permit the achievement of a greater depth sharpness, that is advantageous in the case of the curved cornea. In addition, the energy of the excitation radiation is better utilized than when using a mask, that does not allow a part of this radiation to reach the eye. Furthermore, a more precise sinusoidal intensity course of, e.g., alternating bright and dark strips can be generated than in the case of a mask.
Alternatively, the irradiation pattern can also be produced on the biological tissue by interference in that the radiation, that is widened out, if necessary, is sent from a monochromatic, coherent radiation source (laser) through a beam splitter and is recombined and united on the biological tissue. Alternatively, an interference pattern can be produced on the tissue by two radiation sources coordinated with one another with radiation coherent to one another.
The irradiation pattern can also be produced by a field of micromirrors on which the excitation radiation is reflected to the tissue in a suitable manner. Such mirrors are characterized by short adjustment times and the generation of individual irradiation patterns. For example, 300xc3x97400 micromirrors are arranged uniformly adjacent to each other.
The irradiation pattern produced on the biological tissue can be composed of several partial patterns generated in one or several of the above-mentioned ways.
The structured fluorescent radiation is preferably recorded with a sufficiently sensitive CCD camera with high spatial resolution (high pixel number, e.g., 1280xc3x971024 pixels) that makes possible a site-resolved detection of the fluorescent pattern (a site-resolved detection is especially advantageous on account of the relative simplicity of the evaluation as well as its precision). Several hundred thousand evaluatable data points can be obtained in this manner. In addition, if the excitation radiation is in the UV range and the fluorescent radiation is in the visible range, a CCD camera is also advantageous because its sensitivity in the visible wavelength range is greater than in the ultraviolet range. In addition, the camera lens of customary CCD cameras acts as a band-pass filter for wavelengths above 300 nm, so that fluorescent radiation above this wavelength is detected and not, in contrast thereto, the excitation radiation reflected from the tissue, if the latter is located in the shorter-wave UV range.
The CCD camera can be connected to an amplifier in the case of relatively weak radiation intensities, as well as when using flash lamps.
Advantageous further developments of the invention are set forth in the appended claims.